Washington Securities' website says the company "specialises in the purchase of shares, debentures, investments in frozen funds and other investments". It has a listed P.O Box in the state of Victoria but no company address or contact number.

According to the Australian Securities and Investments Commission, the company was registered on February 19 this year. It has a registered office in Adelaide.

Although last year other Australian companies made a string of low-ball offers to Kiwis, it is believed to be the first time Washington Securities has targeted New Zealand shareholders.

Vector chairman Michael Stiassny said shareholders should seek independent financial advice and check the latest share price on the NZX.

Stiassny said hundreds of Vector shareholders have been caught out in low-ball offers over the past 18 months, many of them "Mum and Dad" investors.

The investors who took the offer have potentially lost hundreds of thousands of dollars in value in total, Vector said.

While Washington Securities' offer was not illegal, Stiassny said it was "unethical".

"We believe our shareholders should be aware of the dangers that exist and the consequences of accepting offers significantly below the market price," he said.

New regulations governing unsolicited offers to investors came into force in December last year.

They put stricter disclosure requirements on those making an offer and forced them to notify issuers of the approach. Under the new rules, shareholders also have up to 10 working days to cancel any sale they have agreed to.

The FMA can order those making an offer to comply with the regulations, with up to a $30,000 fine for breaching the rules.